1. Field of the Invention
The present invention relates to electronic circuits and systems. More specifically, the present invention relates to variable capacitors for use in high power radio frequency applications.
2. Description of the Related Art
High power radio frequency (RF) transistors are ever increasing in power. In the last several years, the power of RF transistors in the cell phone band has doubled approximately every two years. Amplifier circuits often use several of these transistors together to generate thousands or even millions of watts of power. However, the power handling capability of other lumped element components that comprise a high power transistor amplifier (e.g. capacitors, resistors, inductors, etc.) has not increased nearly as rapidly. Therefore, the design of high power RF amplifiers using the newest high power transistors is difficult due to the lack of high power lumped element components. In particular, variable RF tuning capacitors that can handle the required power levels are not available.
In a typical high power RF microstrip amplifier, a 50 ohm input transmission line transitions into transmission lines of much lower impedances (the wider transmission lines). At the transition between different width lines, a shunt capacitor to ground is needed. The low impedance line is then connected to the input (e.g. gate) of the transistors. A similar embodiment is used at the output (e.g. drain) of the transistors.
These types of matching networks are used to transform 50 ohms into the smaller input/output impedance (xcx9c1 ohm) of the transistor. By making the shunt capacitors variable, the matching networks can be tuned. This tuning is necessary to account for differences in transistor performance, circuit board characteristics, and circuit etching tolerances as well as variations in any other lumped element components that my be in the amplifier circuit.
Lumped element variable capacitors are available which can handle up to 200 watts of power. However, if the power output is pushed just a small percentage beyond this, the capacitors will fail. Higher power variable capacitors are available, but they are not suitable at RF frequencies (they self resonate below 900 MHz).
Hence, a need exists in the art for an improved capacitor which can handle the voltage and current levels applied in high power, high frequency applications.
The need in the art is addressed by the variable capacitor of the present invention. In a most general implementation, the inventive capacitor includes a dielectric substrate with a cavity disposed therein. A first conductive surface is mounted on a first side of said cavity and a second conductive surface is mounted on a second side of the cavity. The second conductive surface is mounted so that it is at least partially diametrically opposed to the first conductive surface. In accordance with the present teachings, a dielectric stub is provided which is adapted for reciprocal motion within the cavity.
In a specific implementation, the first conductive surface is a ground plane and the second conductive surface is a microstrip patch. In the illustrative embodiment, the cavity is rectangular with four rectangular side walls, a rectangular top and a rectangular bottom. In the illustrative embodiment, the patch and stub are also rectangular. An electro-mechanical actuator reciprocates the dielectric stub into and out of the cavity to provide a desired amount of capacitance in response to a control system.